Bet Max is a term used both in brick-and-mortar and online casinos, and it refers to an action a player can take. Most often, it is related to a button that can be found in slot machines and video slots.
When designing video slots, software developers often add the Bet Max option. When players press the button, the game automatically bets the maximum number of coins for all active paylines. In some cases, it even activates all the inactive paylines to provide you with the highest amount you can possibly bet.
The reason behind the Bet Max button is to make the lives of high rollers a tad easier. Instead of increasing the settings one by one, you can simply click on the button and make everything more convenient.